The present invention relates to injection moldable phenolic resin compositions, preferably, in nodule, or pellet form and to a process for producing such compositions. The present compositions contain high percentages of filler materials.
Phenolic molding compositions have been available for many years. Generally, such compositions consist of a phenol-aldehyde resin blended with various filler materials. The molding compositions are prepared by blending a one- or two-stage phenol-aldehyde resin with filler material. In one method, the compositions were processed by working the mixture between hot rolls to soften the resin and to obtain a blend of the components. The composition is then cooled, crushed and screened. The product, while useful in some molding operations, was not generally uniform in size nor composition and has been found unsuited to the newer methods of molding thermosetting resin compositions. The newer molding methods are adapted to utilize a resin composition in the physical form of a nodule, or pellet. Such nodules are suitably produced by extrusion processes. The extruded products are usually cylindrical and generally range from about 1/16" to about 1/4" in diameter and from about 1/16" to about 1/4" in length, depending upon the use of the product. The extruded product has a higher density and is essentially uniform in size. Such characteristics are desirable for handling and shipping and are equally desirable for mold loading processes.
The phenol aldehyde resins may be made from phenols such as phenol, m-cresol, m,p-cresol mixtures, cresylic acid, mixtures of phenol and cresylic acid, xylenol, resorcinol, bisphenol A, or any other phenol which will form thermosetting resins with aldehydes. Suitable aldehydes, for example, are formaldehyde, acetaldehyde, benzaldehyde, furfural, propionaldehyde, glyoxal, acrolein and crotonaldehyde. Preferred thermosetting resin is phenolformaldehyde resin, and, more preferably, is a phenol-formaldehyde novolac which includes a cross-linking agent such as hexamethylenetetramine or paraformaldehyde.
Filler materials utilized in molding compositions may be organic or inorganic. Such materials are, primarily, added to enhance the properties of the final molded product and, secondarily, to utilize a less expensive material in place of the more expensive resin material. Examples of inorganic filler materials are metals, metal oxides, asbestos, silica, chopped fiberglass, calcium carbonate, minerals, e.g. wollastonite, talc, and quartz powders, clay, coal, mica, and carbon black. Examples of organic filler materials are rubber, wood flour, cloth fibers, rag pulp, wool and cotton flock. The characteristics of the final cured product, for example, electrical conductivity, moisture resistance, heat resistance and thermal expansion and conductivity, may be modified or improved by the choice and amounts of filler materials.
Although the choice of filler material is broad, the amount of filler material that may be included is limited, due to the increase in viscosity of the mixture as the amount of filler material is increased. Generally, a total filler content between about 20 and about 70 percent by weight of the composition is useful. The increase in viscosity is particularly noted in regard to increases in amounts of inorganic filler materials that are added. The practical limits of loading, i.e. the maximum amount of inorganic filler material that may be added, is usually limited to a maximum of about 40 to about 60 percent by weight. Compositions with amounts of inorganic filler materials above that range are extruded and molded with extreme difficulty. Many inorganic materials, particularly mineral powders, have an irregular particle size and tend to pack at higher loadings, over about 30 to about 40 percent by weight, and such compositions cannot be easily extruded or molded.